Current feedback amplifiers, like operational amplifiers (op-amps) in general, are used extensively in control, computation, and measurement applications. Their principle advantage is that a feedback circuit is used to produce an output signal which is largely free of distortion and non-linearity.
A current feedback amplifier operates by generating an error current in response to a difference between the voltage applied to the amplifier input and a divided portion of the voltage on the amplifier output. The error current is then used to adjust the output voltage of the amplifier in the direction that eliminates this voltage difference. This is accomplished by using a voltage divider feedback circuit in the following manner.
A replica of the input voltage, V.sub.I (ref), is applied to the lower leg of the voltage divider feedback circuit, creating an input reference point. The upper leg of the divider feedback circuit is connected to the amplifier output. For a given output voltage, V.sub.0, error current flows into or out of the feedback circuit through the reference point when V.sub.I (ref) deviates from the divided voltage, V.sub.0 R.sub.G /(R.sub.F +R.sub.G), where R.sub.F and R.sub.G are the impedances in the upper and lower legs of the feedback divider circuit. The divided voltage is just the voltage that would exist at the lower leg of the feedback circuit for a given V.sub.0 if the feedback circuit was disconnected from the reference point.
The error current so generated charges a capacitor which changes V.sub.0. Error current continues to flow until; EQU V.sub.0 =V.sub.I (ref) (1+R.sub.F /R.sub.G), (I)
at which time the error current falls to zero and the output voltage ceases to change. The overall gain or amplification of the current feedback amplifier is determined by the ratio of the impedance in voltage divider feedback circuit.
A major problem with the operation of current feedback amplifiers is that capacitive effects in the amplifier circuit limit the rate at which the output can slew to follow an input. When this occurs, the difference between the divided voltage, V.sub.0 R.sub.G /(R.sub.F +R.sub.G) and V.sub.I (ref) generates error currents which can be large enough to saturate transistors in the input and subsequent stages of the amplifier circuit. If such transistors are saturated, any further increase in the voltage difference between the input and reference point will not generate larger error currents, as should happen for ideal amplifier behavior. Rather, the voltage at the amplifier output will slew at a rate determined by the maximum current of the saturated transistor, whose slew rate will be less than the slew rate of the signal voltage at the amplifier input. As a result, the amplifier will generate an output signal which is distorted relative to the input signal applied to the circuit and will exhibit saturation and recovery transients after the input signal is over.